The present invention relates to an image forming apparatus, such as a laser printer, and which is adapted to form an image using a reusable image carrier.
As shown in FIG. 1, an image forming apparatus, e.g., a laser printer, comprises a photosensitive drum 2 which is rotated in a predetermined direction. Drum 2 is surrounded by main charger 4, exposure section 8 of exposure system 6, developing unit 10, transfer charger 12, separation charger 14, cleaner 16, and de-electrifier 18, which are arranged successively in the rotating direction of drum 2.
First, the surface of photosensitive drum 2 is charged uniformly by main charger 4, and is then exposed by exposure system 6. Thus, an electrostatic latent image is formed on the drum surface. Subsequently, a toner is supplied to the latent image by developing unit 10. As a result, the latent image is visualized or developed into a toner image. Then, the toner image is transferred, by transfer charger 12, to the surface of a paper sheet which is intimately in contact with the surface of drum 2. Thereafter, the sheet is separated from drum 2 by separation charger 14.
Those toner particles remaining on the surface of photosensitive drum 2, without having been transferred to the sheet surface, are removed by means of cleaner 16. Thereafter, the electrostatic latent image on the surface of drum 2 is erased. Thus, one cycle of image formation is finished.
Conventionally, the residual toner particles on the surface of photosensitive drum 2 are scraped off from the drum surface by means of blade 20 attached to cleaner 16. The scraped toner is collected in cleaner 16. Usually, the internal space of cleaner 16 is filled up with the toner after the image is formed on 2,000 to 3,000 sheets. As a result, cleaner 16 becomes unusable.
Image forming apparatuses with the above described construction include ones which are designed so that the disabled cleaner can be discarded together with photosensitive drum 2. In these apparatuses, however, the expendables cost much. In the case of frequently used apparatuses, such as printers, in particular, they will become unusable during the replacement of cleaner 16 and drum 2. Therefore, such apparatuses are not preferred.
In prior art image forming apparatuses, toner conveyor screw 22 for toner recovery is provided within cleaner 16. Screw 22 serves to deliver the toner in cleaner 16 to a toner recovery box (not shown) outside the cleaner. Thus, the toner is recovered.
The recovery box is bound to occupy a certain space inside the apparatus, so that it cannot be large in size. Also, after an image is formed on several thousands of paper sheets, the box must be replaced with a new one. At the time of removal of the box, some of the toner may possibly spill, thereby soiling the operator's hands or clothes, or the floor.
Blade 20 of cleaner 16 is brought into contact with the surface of photosensitive drum 2 so that the drum surface is liable to be scratched thereby. For example, an OPC (organic photoconductor) photosensitive drum is safe and harmless. Due to its softness, however, the drum of such a material can enjoy only a very short life. If photosensitive drum 2 has a short diameter, in particular, it must rotate many times to accomplish recording on each paper sheet. Accordingly, one and the same portion of drum 2 would be used very frequently for each sheet so that the life and the replacement cycles of the drum are inevitably short. It is not advisable, therefore, to use a slender photosensitive drum. Thus, the apparatus cannot be easily reduced in size.
In order to settle these problems, image forming apparatuses without a cleaner have been developed (U.S. patient application Ser. Nos. 571,800 and 901,312; U.S. Pat. Nos. 3,117,884; 3,649,262; 3,997,262; 4,320,958; 4,384,545; 4,426,151; and 4,470,693; and German Patent Publication Nos. 3,006,033 and 3,140,190). In these apparatuses, the developing unit has a cleaning function, that is, development and cleaning are effected simultaneously by means of the developing unit.
In the image forming apparatuses of this type, however, a photosensitive drum is charged by means of a main charger from above those toner particles which remain on the surface of the drum to be transferred therefrom to the surface of a paper sheet by means of a transfer charger. Although some regions of the drum surface are charged, therefore, other regions are not. Thus, the surface potential of the drum is subject to unevenness.